Photovoltaic systems (PV system) utilize solar panels to convert sunlight into electricity. A typical system is made up of one or more solar photovoltaic (PV) panels, a racking system that holds the solar panels, electrical interconnections, and control components. The PV system generally provides electrical energy to a residential or commercial building.
A grid connected or grid-tied PV system is connected to a larger independent grid (typically the utility power grid) and often feed generated electrical power directly into the grid though a portion or all of the generated electrical power may be utilized by the residential or commercial building before being fed to the larger grid. Any excess electrical power fed to the grid may be credited to the owner of the PV system. That is, the feedback is done through a meter to monitor power transferred. Feeding the excess electricity into the grid requires the transformation of DC electricity generated by the PV panels into AC electricity by a special synchronizing grid-tie inverter. The grid-tie inverter converts DC electricity into AC electricity that is matched in phase and frequency with the AC electricity of the utility power grid prior to being fed into the utility power grid. Grid-interactive inverters typically cannot be used in standalone applications where utility power is not available. In this regard, such grid tie inverters require a reference voltage/current from the utility power grid to operate. Grid-tie inverters are also designed to quickly disconnect from the utility power grid if the utility power grid goes down. This disconnect functionality ensures that, in the event of the utility power grid going down (e.g., blackout), the grid tie inverter will shut down to prevent the energy it produces from being fed into the grid which may result in an islanding condition that may potentially harm any line workers who are sent to fix the utility power grid. Stated otherwise, when the utility power grid goes down, grid tied PV systems shut down. However, the use of grid tied system enables use of a PV system without extensive rewiring and without batteries.
Standalone systems do not have a connection to a utility power grid. In stand-alone photovoltaic power systems, the electrical power produced by the photovoltaic panels cannot always be used directly. More typically, all generated electrical power is utilized to charge a battery bank and application loads are connected to the battery bank.
Each of the systems has certain benefits and drawbacks. For instance, grid tied systems avoid the use of costly batteries but, due to the requirement that they shut down if the utility power grid goes out, do not provide energy during utility power loss. Standalone systems can be significantly simplified in relation to the grid-tied systems due to the elimination of synchronizing inverters and other components but require the use of costly battery banks.
In household and commercial usage, most hot water heaters in North America, as well as numerous other countries, are of a storage type. Such storage type water heaters typically include a cylindrical vessel/container (i.e., tank) in which water is kept continuously hot and ready for use. Typical sizes for household use range from about 20 to 120 U.S. gallons. Water heaters for commercial applications are often much larger. Heating the water in the tank is typically affected by way of electrical heating elements or fossil fuel burners (e.g., natural gas, propane, fuel oil, bio mass, etc.) burners. Often, the source of the energy for heating is a local utility.
In most hot water heaters, water is introduced into the storage tank via a water supply inlet pipe. Water typically enters residences in the US at about 50° F. though this varies with latitude of the residence and the season. An electrical element within the tank or a burner disposed beneath the tank is selectively operated to heat the water within the tank to a preset temperature. For instance, hot water temperatures of 105-120° F. are typically preferred for residential applications. Operation of the element or burner is controlled by a thermostat that monitors/measures the temperature within the tank. When the water within the tank is below a desired temperature, the electrical element is energized or the burner is ignited to heat the tank and the water therein.
Energy efficiencies of water heaters in residential use can vary greatly based on manufacturer, model and/or age. Gas fired water heaters typically have efficiencies of between about 55% and 95%. Electric water heaters have high efficiencies (e.g., above 95%) once the electricity enters the home but are more costly to operate due to electricity generation and transmission costs. In either case, a typical household expends roughly 30% of its total energy use heating water.